Birds Descended from Gliding Dinosaurs

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Evidence is mounting that modern birds descended from gliding,
feathered non-avian dinosaurs.

Two dinosaurs could be candidates for the bottom of the bird
family tree, and each helps to reveal how feathers first evolved.

"The oldest known feathered dinosaurs would be
Anchiornis (155 million years ago) and
Epidexipteryx (between 152 million and 168 million years
ago)," Yale University paleontologist Nicholas Longrich told
Discovery News. "Feathers seem to have appeared initially for
insulation. Basically they start out as down, and later are used
to make wings."

For a study published in the latest Current Biology, Longrich and
colleagues Jakob Vinther and Anthony Russell examined fossils of
Anchiornis huxley and of Archaeopteryx
lithographica, a Jurassic species that could be the world's
oldest known bird.

"Where dinosaurs end and birds begin is a bit arbitrary,"
Longrich explained. "There's no clear cutoff that separates one
from the other. That's the nature of evolution; things gradually
change from one thing into another."

The scientists found that the wing feathers of
Archaeopteryx and Anchiornis were similar, but
not identical. The variations between the two appear to represent
early experiments in the evolution of the wing.

Archaeopteryx had multiple layers of long flight
feathers. In contrast, the dinosaur Anchiornis had an
abundance of simple, strip-like feathers that overlap, somewhat
similar to the feathers on penguins.

The design and arrangement of Anchiornis and
Archaeopteryx wing feathers probably hindered liftoff.
Multiple overlapping layers of long wing feathers would have
complicated feather separation, minimizing the bird's ability to
overcome drag on the upstroke. By contrast, the wings of modern
flying birds typically have a single primary layer of easily
separated long feathers overlain by short feathers.

"Modern birds have the ability to separate their wing feathers
sort of like a Venetian blind," Longrich said. "This allows them
to raise the wing rapidly, and seems to be critical to flapping
flight at low speeds."

"The feather arrangement in Archaeopteryx and
Anchiornis wouldn't let them do this," he added, "so it
may have made takeoff from the ground and flapping at low speeds
more difficult."

Gliding, however, must have been a lifesaver back in the dinosaur
day, when huge terrestrial carnivores were stomping around.

"Gliding is a fast way to move from tree-to-tree. Instead of
climbing down one tree and running up the next, you just glide
quickly from one to the other," Longrich explained.

"I would imagine that the dinosaurian ancestors of birds were
living in the trees," he noted, "probably to find food-like
insects, lizards and mammals, and to avoid becoming food for
other dinosaurs."

Longrich and his colleagues believe that the wing feather
arrangement seen in modern birds may have evolved within a period
spanning a few tens of millions of years and then remained
largely unchanged for the past 130 million years.

In terms of this short versus long timescale, Longrich compared
it to the evolution of human-constructed aircraft, which started
with some years of experimentation before settling into a basic
design that's just been fine-tuned during more recent years.

"Birds hit on a workable design about 130 million years ago, and
it's been difficult to improve upon it," he said.

Birds benefitted from both this and their small size 65 million
years ago, when the larger non-avian dinosaurs bit the dust.

Xu Xing, one of the world's leading paleontologists, is a
professor at the Institute of Vertebrate Paleontology and
Paleoanthropology at the Chinese Academy of Sciences. He told
Discovery News that the determination about ancient versus modern
feathers and wings "is definitively a significant discovery."

The study "has greatly improved our understanding of wing
evolution," Xu continued. "(The authors) demonstrate that there
are unusual wings near the dinosaur-bird transition, though more
data is needed to confirm the claim that it is directly related
to the origins of avian flight."